[codeview] Try to avoid emitting .cv_loc with line zero

Summary:
Visual Studio doesn't like it while stepping. It kicks you out of the
source view of the file being stepped through and tries to fall back to
the disassembly view.

Fixes PR43530

The fix is incomplete, because it's possible to have a basic block with
no source locations at all. In this case, we don't emit a .cv_loc, but
that will result in wrong stepping behavior in the debugger if the
layout predecessor of the location-less BB has an unrelated source
location. We could try harder to find a valid location that dominates or
post-dominates the current BB, but in general it's a dataflow problem,
and one still might not exist. I left a FIXME about this.

As an alternative, we might want to consider having the middle-end check
if its emitting codeview and get it to stop using line zero.

Reviewers: akhuang

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68747

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374267 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp b/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
index 616b1368a7a75fdba830fe306892c69e1527163e..012da940f899512f776b30826585b7f7e37620f4 100644 (file)
--- a/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
+++ b/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
@@ -2858,6 +2858,14 @@ void CodeViewDebug::endFunctionImpl(const MachineFunction *MF) {
   CurFn = nullptr;
 }
 
+// Usable locations are valid with non-zero line numbers. A line number of zero
+// corresponds to optimized code that doesn't have a distinct source location.
+// In this case, we try to use the previous or next source location depending on
+// the context.
+static bool isUsableDebugLoc(DebugLoc DL) {
+  return DL && DL.getLine() != 0;
+}
+
 void CodeViewDebug::beginInstruction(const MachineInstr *MI) {
   DebugHandlerBase::beginInstruction(MI);
 
@@ -2869,19 +2877,21 @@ void CodeViewDebug::beginInstruction(const MachineInstr *MI) {
   // If the first instruction of a new MBB has no location, find the first
   // instruction with a location and use that.
   DebugLoc DL = MI->getDebugLoc();
-  if (!DL && MI->getParent() != PrevInstBB) {
+  if (!isUsableDebugLoc(DL) && MI->getParent() != PrevInstBB) {
     for (const auto &NextMI : *MI->getParent()) {
       if (NextMI.isDebugInstr())
         continue;
       DL = NextMI.getDebugLoc();
-      if (DL)
+      if (isUsableDebugLoc(DL))
         break;
     }
+    // FIXME: Handle the case where the BB has no valid locations. This would
+    // probably require doing a real dataflow analysis.
   }
   PrevInstBB = MI->getParent();
 
   // If we still don't have a debug location, don't record a location.
-  if (!DL)
+  if (!isUsableDebugLoc(DL))
     return;
 
   maybeRecordLocation(DL, Asm->MF);

diff --git a/test/DebugInfo/COFF/line-zero.ll b/test/DebugInfo/COFF/line-zero.ll
new file mode 100644 (file)
index 0000000..3d34345
--- /dev/null
+++ b/test/DebugInfo/COFF/line-zero.ll
@@ -0,0 +1,77 @@
+; RUN: llc < %s | FileCheck %s
+
+; C++ source to regenerate:
+; int main() {
+;   volatile int x;
+;   x = 1;
+; #line 0
+;   x = 2;
+; #line 7
+;   x = 3;
+; }
+
+
+; CHECK-LABEL: main:                                   # @main
+; CHECK:         .cv_loc 0 1 1 0                 # t.cpp:1:0
+; CHECK:         .cv_loc 0 1 3 0                 # t.cpp:3:0
+; CHECK:         movl    $1, 4(%rsp)
+; CHECK-NOT: .cv_loc {{.*}} t.cpp:0:0
+; CHECK:         movl    $2, 4(%rsp)
+; CHECK:         .cv_loc 0 1 7 0                 # t.cpp:7:0
+; CHECK:         movl    $3, 4(%rsp)
+; CHECK:         .cv_loc 0 1 8 0                 # t.cpp:8:0
+; CHECK:         xorl    %eax, %eax
+; CHECK:         retq
+
+; ModuleID = 't.cpp'
+source_filename = "t.cpp"
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc19.22.27905"
+
+; Function Attrs: norecurse nounwind uwtable
+define dso_local i32 @main() local_unnamed_addr #0 !dbg !8 {
+entry:
+  %x = alloca i32, align 4
+  %x.0.x.0..sroa_cast = bitcast i32* %x to i8*, !dbg !15
+  call void @llvm.dbg.declare(metadata i32* %x, metadata !13, metadata !DIExpression()), !dbg !15
+  store volatile i32 1, i32* %x, align 4, !dbg !16, !tbaa !17
+  store volatile i32 2, i32* %x, align 4, !dbg !21, !tbaa !17
+  store volatile i32 3, i32* %x, align 4, !dbg !22, !tbaa !17
+  ret i32 0, !dbg !23
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #2
+
+attributes #0 = { norecurse nounwind uwtable }
+attributes #1 = { argmemonly nounwind willreturn }
+attributes #2 = { nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "t.cpp", directory: "C:\5Csrc\5Cllvm-project\5Cbuild", checksumkind: CSK_MD5, checksum: "8b6d53b166e6fa660f115eff7beedf3b")
+!2 = !{}
+!3 = !{i32 2, !"CodeView", i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 2}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!7 = !{!"clang version 10.0.0"}
+!8 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !9, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !12)
+!9 = !DISubroutineType(types: !10)
+!10 = !{!11}
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!12 = !{!13}
+!13 = !DILocalVariable(name: "x", scope: !8, file: !1, line: 2, type: !14)
+!14 = !DIDerivedType(tag: DW_TAG_volatile_type, baseType: !11)
+!15 = !DILocation(line: 2, scope: !8)
+!16 = !DILocation(line: 3, scope: !8)
+!17 = !{!18, !18, i64 0}
+!18 = !{!"int", !19, i64 0}
+!19 = !{!"omnipotent char", !20, i64 0}
+!20 = !{!"Simple C++ TBAA"}
+!21 = !DILocation(line: 0, scope: !8)
+!22 = !DILocation(line: 7, scope: !8)
+!23 = !DILocation(line: 8, scope: !8)

diff --git a/test/DebugInfo/COFF/local-variables.ll b/test/DebugInfo/COFF/local-variables.ll
index e15152050d811b60eed72214e8fba88224f7c63b..5e96160f189bc8d3beff5b38c04320fed2763d72 100644 (file)
--- a/test/DebugInfo/COFF/local-variables.ll
+++ b/test/DebugInfo/COFF/local-variables.ll
@@ -60,7 +60,7 @@
 ; ASM:         leaq    36(%rsp), %rcx
 ; ASM: [[else_end:\.Ltmp.*]]:
 ; ASM: .LBB0_3:                                # %if.end
-; ASM:         .cv_loc 0 1 0 0                # t.cpp:0:0
+; ASM:         .cv_loc 0 1 17 1                # t.cpp:17:1
 ; ASM:         callq   capture
 ; ASM:         nop
 ; ASM:         addq    $56, %rsp